Yeah, that's a great free service. I've even tried it on a youtube movie conversion (flv, or was it fla? whatever) to avi.
I don't know what file format you're starting with, but you might be able to convert with the gimp as well
http://www.gimp.org/.
If it's just a jpg or gif or something, and you want to convert to a tga, the gimp would probably be easier to use if you have to do this sort of conversion often.